rndis_wlan: make some variables unsigned

These variables can never be less than zero because we cap them in
get_device_pmkids().  Let's make them unsigned here because it's simpler
to not have to worry about negative numbers when we read the code.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>